Field of the Invention
The invention relates to a once-through steam generator having a first gas flue followed on the fuel-gas side, by way of a horizontal gas flue, a second gas flue. A steam generator of this type is known from Published, European Patent Application EP 0 308 728 A1.
In a once-through steam generator, the heating of a number of evaporator tubes, which together form the gas-tight containing wall of a combustion chamber, leads to complete evaporation of the flow medium in the evaporator tubes in a single pass. The flow medium, usually water, after evaporating, is supplied to superheater tubes located downstream of the evaporator tubes and is superheated there. In contrast to a natural-circulation steam generator, a once-through steam generator is not subject to any pressure limitation, so that fresh steam pressures well above the critical pressure of water (P.sub.crit =221 bar), where there is still only a slight difference in density between a liquid-like and a steam-like medium, are possible. A high fresh steam pressure is condusive to high thermal efficiency and therefore low CO.sub.2 emissions of a fossil-fired power station.
A once-through steam generator of this type can have a single-flue configuration or else a double-flue configuration.
In the case of a once-through steam generator of the single-flue configuration, the steam generator tubes are usually welded to one another in a gas-tight manner in order to form the containing wall of a single gas flue, the gas flue being disposed vertically. In this case, as a rule, the steam generator tubes forming the containing wall of the gas flue contain both evaporator tubes and superheater tubes located downstream of these on the flow-medium side. A combustion chamber with a number of burners for fossil fuel is usually provided in a lower region of space of the gas flue.
In the case of a once-through steam generator having a double-flue configuration, steam generator tubes are likewise usually welded to one another in a gas-tight manner in order to form the containing wall of a vertically disposed first gas flue. In this configuration, however, the first gas flue is followed by way of a horizontal gas flue, on the fuel-gas side, by a second vertically disposed gas flue, the containing wall of which is likewise formed by steam generator tubes and through which the fuel gas normally flows from the top downwards. A once-through steam generator having a double-flue configuration usually has a lower overall height, as compared with a once-through steam generator having a single-flue configuration, and differs from this in a number of configuration parameters.
In a once-through steam generator having the double-flue configuration, the steam generator tubes forming the containing wall of the first gas flue are normally configured as evaporator tubes, whereas steam generator tubes configured as superheater tubes are part of the containing wall of the second gas flue and/or part of a wall heating surface of the horizontal gas flue. In other words, the steam generator tubes assigned to the horizontal gas flue and those assigned to the second gas flue are usually located downstream, on the flow medium side, of the steam generator tubes assigned to the first gas flue. For this purpose, the steam generator tubes assigned to the first gas flue open on the outlet side into an outlet header which is common to them and which is followed, by way of a water/steam separating device and by way of a number of heating surfaces disposed in a horizontal gas flue, by an inlet header for the steam generator tubes assigned to the second gas flue.
In the once-through steam generator known from the Published, European Patent Application EP 0 308 728 A1, a number of steam generator tubes connected in parallel for a flow medium to flow through them are connected to one another to form an evaporator heating surface which is part of the containing wall of the first gas flue. In this case, the steam generator tubes forming the evaporator heating surface open on the outlet side into an outlet header which is common to them and which is disposed at a lower height, as compared with a bottom edge of the horizontal gas flue.
In a configuration of this type, particularly during start-up, also referred to as hot start-up, after a comparatively short shutdown time prior to the ignition of the burners, when steam generator tubes of the still hot once-through steam generator are being filled with cold feed water, considerable temperature differences may occur between the steam generator tubes assigned to the first gas flue and steam generator tubes assigned to a containing wall of the horizontal flue. Temperature differences of this kind may give rise to inadmissible thermal stresses, particularly at a connection point at which the containing wall of the first gas flue is welded to a wall of the horizontal flue. Due to thermal stresses of this kind, the lifetime of the once-through steam generator of this type is only limited because of high alternating stress, particularly in the case of frequent start-up operations. In this case, the thermal stresses occur particularly after only a short shutdown of the once-through steam generator, that is to say, for example, after a night-time shutdown, since the once-through steam generator then normally still has a temperature which is high in comparison with the temperature of the feed water.